Astronomy - USA (2019-09)

WWW.ASTRONOMY.COM 45

Life in the

SOLAR

SYSTEM

drain into liquid-filled basins, some as

large as the terrestrial seas. Titan’s largest

sea, Kraken Mare, covers about 154,000

square miles (400,000 square kilometers),

making it roughly five times the area of

North America’s Lake Superior, or nearly

the size of Asia’s Black Sea.

A second kind of precipitation may

have even more bear-

ing on the search for

life on Titan: a steady

drizzle of hydrocar-

bons. This organic

“soot” combines with

methane to form complex

compounds, the raw mate-

rials of life. Titan’s winds pile

up the hydrocarbon fallout

into vast sand seas.

But methane and hydrocarbons aren’t

the only things falling from Titan’s soupy

skies. Sunlight and radiation from Saturn

break up nitrogen and methane molecules

in Titan’s atmosphere. When these frag-

ments recombine, they create a com-

pound called vinyl cyanide. Vinyl cyanide

is important in the search for life because

it tends to assemble into membranes like

those found in terrestrial living cells.

This news has exciting implications

for life on Titan, but it was a long time

coming. Over a decade ago, the

Cassini spacecraft detected the

building blocks of vinyl cya-

nide on the distant moon,

yet it was not equipped to

confirm its exis-

tence. Later,

in 2014,

astrono-

mers cali-

brating the

66 anten-

nas of the Atacama

Large Millimeter/

submillimeter

Array in Chile hap-

pened to use Titan

as their target. As

luck would have it,

their data contained

the fingerprints of

vinyl cyanide. We now know that thou-

sands of tons of the stuff f loat high in

Titan’s upper atmosphere, and an

astounding 10 billion tons may have

accrued in Titan’s largest methane seas,

Ligeia Mare and Kraken Mare.

The ability to form cell membranes is

certainly not a guarantee of life. However,

it is likely one of the prerequisites. Plus,

Titan has even more to offer, says plan-

etary scientist and engineer Ralph

Lorenz of Johns Hopkins University’s

Applied Physics Laboratory, who was

part of the team that landed Cassini’s

Huygens probe on Titan. Lorenz believes

Titan is interesting for several key rea-

sons. “There are processes going on there

that we don’t see at Mars today: the for-

mation of clouds and rain, the pooling of

liquids on the surface, their movement

by tides and presumably by wind as well,

the formation of waves. You have a much

more astrobiologically interesting chem-

istry at Titan because of the methane

being processed into literally hundreds

of other compounds,” he says.

WITH A

DIAMETER of

about 3,200 miles

(5,150 kilometers),

Titan is wider

than Mercury. This

natural-color mosaic

of the moon passing

in front of Saturn’s disk

was obtained by the

Cassini spacecraft

in 2012. NASA/JPL-CALTECH/

SPACE SCIENCE INSTITUTE

AS

TR

ON

OM

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RO

EN

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Y

Hydrogen

Hydrogen

Nitrogen

Carbon

VINYL

CYANIDE

Chemical formula:

C 2 H 3 CN

First detection in

space: 1975

First clear detection

on Titan published:

2017

Estimated availability

in Ligeia Mare alone:

Enough for 10 million

cell membranes per

cubic centimeter